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E439: A new target for Archean paleomagnetism (Lead Supervisor: Richard Harrison, Earth Sciences)

Supervisors: Richard Harrison (Earth Sciences), Rich Taylor (Earth Sciences) and Charan Kuppili (University of Cambridge)

Importance of the area of research:

Paleomagnetic measurements demonstrate that the geodynamo was active at least 3.5 billion years ago and that the intensity of the magnetic field was ~50-70% of the present-day field. There are currently no undisputed paleomagnetic data that predate 3.5 Ga, meaning that the behaviour of the geodynamo for the first one billion years of Earth's history remains unknown. Recent attention has focussed on Hadean zircon crystals from the Jack Hills, which have been proposed to contain the only record of the Hadean geodynamo. However, the ability of zircon to trap and retain primary magnetite grains has not yet been demonstrated unequivocally, and our recent work suggests that zircon may not be the most ideal host for primary magnetite after all. The search is on, therefore, for alternative targets for single crystal paleomagnetic studies. A potential target (target X) has recently been identified, which has a much greater preservation potential for ideal primary magnetic remanence carriers than zircon. If this early promise can be delivered upon, it has the potential to make vast swathes of the Hadean and Archean accessible to paleomagnetic analysis for the first time.

Project summary:

The project will apply state-of-the-art characterization tools to evaluate the morphology, structure, chemistry, redox state and magnetic properties of iron oxide nanoparticles in target X. The proposed project will focus on the mineralogy of magnetic inclusions from a range of Archean rocks. Our chief aims are a) to understand the formation mechanism of magnetic inclusions in target X, and develop methods to distinguish between primary and secondary inclusions, b) to explore the potential of these inclusions as a paleomagnetic recorders, c) to constrain how and when they became magnetised, and to develop both experimental and computational methods to enable their paleomagnetic properties to be measured and interpreted. Primarily the work will focus on new correlative methods of 3D mineralogical analysis linking behaviour across multiple length scales (atomic to micrometre).

What the student will do:

The student will use a variety of methods to explore the potential of target X as a paleomagnetic archive in ancient rocks. They will use slice-and-view tomography to determine the 3-dimensional make up of the inclusions at the nm- µm scale. This involves taking successive ~20 nm slices through the crystals using a focussed ion beam (FIB), acquiring detailed chemical and crystallographic information from each slice. The 3D structure will be explored at the atomic scale using atom-probe-tomography, whereby a 3D volume of the sample is reconstructed atom-by-atom. Finally, the 3D information will be used to create computer models that accurately describe the paleomagnetic behaviour of these intergrowths and determine how and when they became magnetised. The student will also use the new mineral to demonstrate its suitability in a range of samples spanning geological time. The student will undertake fieldwork in multiple sites across the UK including the Archean rocks of NW Scotland. The global significance of the results from these UK samples will be investigated with comparative studies of samples obtained from other Archean terrains in Greenland and Western Australia.

Please contact the lead supervisor directly for further information relating to what the successful applicant will be expected to do, training to be provided, and any specific educational background requirements.

References:

Tarduno, J.A., Cottrell, R.D., Davis, W.J., Nimmo, F., Bono, R.K., 2015. A Hadean to Paleoarchean geodynamo recorded by single zircon crystals. Science (80-. ). 349, 521-524.

Einsle, J.F., Harrison, R.J., Kasama, T., Conbhuí, P.Ó., Fabian, K., Williams, W., Woodland, L., Fu, R.R., Weiss, B.P., Midgley, P.A., 2016. Multi-scale three-dimensional characterization of iron particles in dusty olivine: Implications for paleomagnetism of chondritic meteorites. Am. Mineral. 101, 2070–2084.

Fu, R.R., Weiss, B.P., Lima, E.A., Kehayias, P., Araujo, J.F.D.F., Glenn, D.R., Gelb, J., Einsle, J.F., Bauer, A.M., Harrison, R.J., Ali, G.A.H., Walsworth, R.L., 2017. Evaluating the paleomagnetic potential of single zircon crystals using the Bishop Tuff. Earth Planet. Sci. Lett. 458, 1–13.

Follow this link to find out about applying for this project.

Other projects available from the Lead Supervisor can be viewed here.

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